Why is it not letting me claim my child tax credit

You need to review the information that you have entered for your child to be sure that they qualify as your dependent on your tax return.  If so, and the child tax credit qualifications are met, then the child tax credit is automatically included as part of your return.  

Take a look at the following TurboTax help article to learn more about the child tax credit qualifications:

What is the Child Tax Credit?
 

 There are 7 possible reasons; you aren’t getting the Child Tax credit (CTC). It’s usually #4:

1. You’ve entered something wrong. In the personal Info section, for the dependent, you must select answers that indicate that he/she is your dependent child. If the child was born during the year, say he/she lived with you all year (note: TurboTax changed how this section is done two years ago, you may need to go thru the interview again or even delete your dependent and start over). If the child was with you for 6 months and 1 day, you need to mark that the child was with you 7 months. The CTC is not an automatic $2000, per child. It is income dependent. If you haven't entered your income yet, the CTC will not show.
2. Your child may be  too old (over 16). You can still claim your child, as a dependent. What you can't claim is the Child tax credit. This comes as a big surprise to many parents the year their child turns 17. A child over age 16 no longer qualifies for the Child Tax credit (CTC). Although a child can still be a student dependent through age 23, and a qualifying child for EIC,  the Child Tax Credit expires the year they turn 17 and you no longer get the $2000 CTC. Instead you will get the non-refundable (up to) $500 Other Dependent Credit.
3. Your income is too high. The Child Tax Credit (CTC) is phased out at higher incomes starting at $400,000 for joint filers  ($200K single). You lose $50 for each $1000 (rounding up) your income is over that threshold.   Previously it was $75,000 for single, head of household, and qualifying widow or widower filers; and $110,000 for joint filers.
4. Your income is too low.  The child tax credit  (CTC) is limited to your tax liability. The CTC is a non-refundable credit and can only reduce your income tax to 0, It can not help you beyond eliminating your tax liability. But, if you have more than $2500 of earned income, some or all of it is usually given back to you thru the "Additional Child tax credit". That is, part of the CTC may be on line 28 of form 1040 (2021- 2024) instead of line 19. The  ACTC is calculated on form 8812 and  is basically 15% of your earned income over $2500. The ACTC is a maximum of $1700 (2024) ($1600 for 2023, $1500 for 2022) per child, not $2000.
5. You are the custodial parent and the non-custodial parent is claiming the dependent this year. The CTC goes with the dependency, even though the custodial parent still gets the Earned Income Credit, Dependent care credit and Head of Household.
6. Another possibility is that part of your tax due is not regular income tax, but is self-employment tax (FICA), early distribution penalty or another type of additional tax, for which the CTC cannot be used.
7. During covid (2021), the child tax credit was expanded and easier to get. That was temporary and those relaxed benefit rules are no longer available.

To get a 'second opinion' on-line direct from IRS, try https://www.irs.gov/uac/is-my-child-a-qualifying-child-for-the-child-tax-credit